Cephalosporin and penicillin antibiotics having various substituents attached to the C.sub.7 and C.sub.6 atoms of the .beta.-lactam ring, respectively, are known. For example, 7-alkylthiocephalosporins and 6-alkylthiopenicillins are described by Spitzer and Goodson in Tet. Let. 273 (1973).
According to the method described therein, 7-acylamino-7-alkylthiocephalosporanic acids 7-acylamino-7-alkylthiodeacetoxycephalosporanic acids and 6-acylamino-6-alkylthiopenicillanic acids and esters thereof are prepared by reacting a benzylimine, prepared from a 6-aminopenicillin or a 7-aminocephalosporin compound and a substituted benzaldehyde with a strong base to generate the anion at C.sub.6 of a penicillin or C.sub.7 of a cephalosporin which is then treated with a thioalkylating agent.